The electronic and manufacturing issues associated thin film transistors (TFTs) can be viewed as representative of the electronic and manufacturing issues associated with many thin film components. TFTs are widely used as switching elements in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof. In typical applications of a thin film transistor, the desire is for a switch that can control the flow of current through the device. It is usually desired that when the switch is turned on a high current can flow through the device. The extent of current flow is related to the semiconductor charge carrier mobility. When the device is turned off, it is desired that the current flow be very small. The ratio between current flow in the on state and current flow in the off state is related to the native charge carrier concentration. It is further desired that the device remain unchanged during operation. The stability of transistors is typically evaluated by holding the device under a constant stress (or bias) that is consistent with the stress applied to the transistor in operation for a given application.
Many electronic devices benefit from the presence of either a passivation layer or a barrier layer or both. Thin film metal oxide TFTs, such as ZnO, GIZO, or GZO, have instabilities that can limit their adoption in practical applications. There has been a concerted effort recently to improve the stability of these types of TFTs with passivation layers. Typical passivation layer strategies employ inorganic thin films, such as Al2O3, as a passivation layer over all devices on a substrate. The use of these uniform inorganic passivation layers typically induces an uncontrolled negative threshold shift that can be undesirable. Complicated processing schemes have been introduced to passivate with inorganic materials without threshold shifts. Alternatively, researchers have used multilayer channels to modify the charge on the back-channel, for instance using two different stoichiometries of IGZO for the semiconductor layer. There has been limited work done to passivate inorganic TFTs with photopatternable polymers, with varied response. In most cases, a negative shift in threshold voltage is still present with passivation and the processing involves the complex multistep process associated with photolithography and additional post deposition annealing steps. There remains a need for a passivation process for metal oxide transistors which is simple, and which results in TFTs stable under bias stress with a desired threshold voltage.
Furthermore, it is recognized in the art that the material that is in contact with the back-channel of a semiconductor has an impact on the performance of the transistor. In the aforementioned cases, the passivation layer is deposited on the back-channel of a bottom gate device. In other architectures, controlling the back-channel interface is still important even when the material layer does not impact the environmental stability of the device. For instance, in the case of top gate TFTs it has been observed that ZnO-based transistors built on glass have very negative threshold voltages. There remains a need for device structures and material layers that control the back-channel interface in all types of device architectures including bottom gate transistors, top gate transistors, and vertical transistors.
A particularly useful electronic device in building functional circuitry is an inverter, which functions to invert the polarity of an input signal. In complementary metal-oxide semiconductor (CMOS) circuitry, inverters are typically easy to design but disadvantageously expensive to produce and utilize complicated production processes. It is possible to build all NMOS or all PMOS inverters. However, particularly for enhancement-depletion mode circuits there are challenges to independently controlling the behavior of each transistor in the inverter circuit. Typically, the depletion mode transistor will have a thicker semiconductor layer than the enhancement mode transistor, increasing process complexity and increasing cost.
There remains a need to build high quality inverters and other circuit elements using simple processes, by employing novel architectures to control transistor, and therefore, circuit performance. Furthermore, there still remains a need for these circuit elements to have high-quality passivation and back-channel control layers that result in stable, high-quality circuits and that can be formed with simple processing methods.